The present invention relates to an extender lens apparatus used for an optical apparatus such as a digital still camera, a video camera, and the like.
A variety of front converter systems have been proposed in which the focal length of an image-pickup lens, which is an image-forming optical system installed in an optical apparatus such as an image-pickup apparatus, is lengthened or shortened by attaching a converter lens to an object side of the image-pickup lens (for example, refer to Japanese Patent Laid-Open No. 2005-331851).
The front converter system is capable of easily changing the range of a focal length of an image-pickup lens due to its simple constitution. However, the converter lens is often large, and as a result, the entire image-pickup lens to which the converter lens is attached becomes large. Further, automatic attachment and detachment of a large converter lens is also difficult.
In contrast to such a front converter system, Japanese Patent Laid-Open No. 4-355709 discloses a built-in extender system. In the built-in extender system, an extender lens is detachably attached (removably inserted) to one of spaces between plural lens units which constitute an image-pickup lens to change the range of the focal length of the entire system of the image-pickup lens.
The extender lens can be configured to be comparatively compact. Therefore, the entire image-pickup lens will not become very large even when the extender lens is attached. Thus, automatic insertion and removal of the extender lens is also easy to carry out.
However, when a built-in extender system is used in an image-pickup lens in which the compactness is valued, the extender lens must be inserted into a rather narrow space. As a result, since the power of the extender lens increases, its optical performance is deteriorated. Since a sufficiently wide space must be provided within the image-pickup lens in order for the built-in extender system to secure sufficient magnification and optical performance, the image-pickup lens becomes large.
Furthermore, Japanese Patent Laid-Open No. 2-184811 discloses one example of the built-in extender system in which lens units are added to plural locations in an image-pickup lens, the position of an image plane is varied in order to lessen the power of each lens unit, and a lens unit closest to an object of the image-pickup lens is moved to correct the position of the image plane. According to this, the built-in extender can be used in an the image-pickup lens which does not have a wide space between the lens units.
However, in the built-in extender system disclosed in Japanese Patent Laid-Open No. 2-184811, although it is not necessarily necessary to provide a wide space to one place in the image-pickup lens, it is necessary to greatly vary the location of the image plane in order to keep the power of the lens unit closest to the image plane small. Further, the lens unit closest to the image plane is moved in order to correct the position of the varied image plane. This configuration is not preferred in optical apparatuses in which compactness is valued.